This invention relates to a fluid pump, and more particularly to a fluid pump adapted for providing a substantially uniform, and steady fluid flow rate.
Although the following description is provided in relation to the application of a fluid pump for hydraulic actuation of domestic elevators, it will be instantly appreciated from the description of the invention provided hereinafter that the invention has wide application where there is requirement for controlled pumping, steady flow during pumping of fluid, and where the hydraulic actuation to be effected by the pump in connection with an hydraulic device is such that there is displacement of the device when the pump acts in both the forward and reverse directions, the interconnection of the pump with the device being such as to form a closed system.
Furthermore, the invention has application in any mechanical action which requires a single positive movement of a prescribed volume of oil or other hydraulic fluid over a given time with a stated load, controlled by fluid movement with electric or electronic sequencing and determining of action.
Domestic elevators are currently driven by reciprocating pumps which urge oil to flow into an hydraulic cylinder which raises or lowers the elevator. The pump is usually submerged beneath an oil reservoir provided in a sealed tank. A large volume of oil is required because as described in our co-pending application of even date, the hydraulic cylinder is substantial for reasons of safety, and accordingly large in capacity. Furthermore the pressure at which the oil must be urged into the cylinder is also significant, and therefore the tank with pump provided therein is an unmanageable and weighty item.
Reciprocating pumps have a number of disadvantages. Firstly, the reciprocating actuation of the pump often introduces harmonic pulses into the fluid as it passes through the impeller of the pump. In their application to domestic elevators, this inherent drawback of reciprocating pumps restricts the particular types of fluid which can pumped therethrough to fluids which are compressible such that the harmonic pulses are damped by the fluid itself and are not transmitted through the fluid to the hydraulic cylinder to which the fluid is supplied. In other circumstances, the use of reciprocating fluid pumps is excluded for this reason. Furthermore, the harmonic pulses within the hydraulic fluid are often manifested in noise within the elevator cabin, which therefore requires soundproofing to , reduce the noise within.
Secondly, reciprocating pumps like reciprocating machinery in general are extremely noisy, and consumer distaste for noise and stringent noise limits in residential areas often necessitates that the tank with pump provided therein is located remotely from the shaft within which the elevator travels, often in a separate building whereby the noise produced can be contained.
Thirdly, the compressibility of oil necessitates a valve/venting arrangement which controls the flow rate oil into the cylinder. This is essential when it is considered that the compression of the oil within the cylinder is often significant enough to result in an appreciable increase or decrease in the height of the elevator cabin adjacent a particular floor when persons alight from or step into the cabin. Such a valve arrangement or other suitable fluid compression compensation system is commonly employed to prevent the unnecessary alarming of passengers within the elevator. The problem is further exacerbated by the fact that the compressibility characteristics of oil depend on its ambient temperature. In many cases, complex electronics is required to control a compensating mechanism which communicates with the pump motor to control the flow rate of oil into and from the cylinder to ensure that the elevation of the cabin above a certain datum level is not adversely affected by a sudden increase or decrease in the load carried by the cabin, and also to ensure that the acceleration and deceleration of the cabin proximate a floor are gradual and not rapid in order that passengers within the elevator cabin are not jolted.
Fourthly, the efficiency of reciprocating pumps is notoriously poor due to frictional losses both within the reciprocating mechanism and at the interface between the impeller of the pump and the fluid, and also because of significant heat and noise generation.
In many applications and currently in the field of domestic elevators as mentioned above, the hydraulic fluid is commonly oil. The flammability of oil presents a fire hazard, especially in domestic environments, and therefore an effectively sealed system is required to prevent leakage of oil. This increases the cost of the apparatus as a whole.
It is the object of this invention to overcome the above disadvantages and provide a pump which provides a steady flow of liquid under pressure to the apparatus to which the pump is connected.
It is a further object of the invention to provide a pump which is capable of pumping any of a number of compressible and substantially incompressible liquids without imparting harmonic pulses to the liquid during pumping thereof.
According to a first aspect of the present invention there is provided a pump for reversible hydraulic actuation of a device, said device and said pump together forming a closed hydraulic system having a fluid therein, said pump comprising a cylinder within which is disposed a piston capable of linearly translating within said cylinder, said pump further having a fluid outlet in communication with the device such that actuation of the device is effected by transfer of fluid between the pump and device when the piston is withdrawn from or urged into the cylinder, said piston being connected to one or other of a ball screw race and spindle engaging said race either of which may be rotationally driven to cause linear motion of the piston, characterised in that the fluid causing actuation of the device is retained in a cavity defined by inner walls of said cylinder, the piston shaft, a first seal between the cylinder and the piston shaft disposed towards the base of the cylinder and a second seal provided on the piston towards a free end thereof which sealingly contacts the inner cylinder walls, said fluid outlet being located on the cylinder between said seals proximate said first seal to allow for expulsion of said fluid through said outlet from said cavity when the piston is withdrawn from said cylinder.
Preferably the spindle is rotationally driven and the ball screw race is connected to the piston such that as the race translates along the spindle, the piston is translated within the cylinder.
Preferably the seals prevent fluid escaping.
In a most preferred embodiment the cylinder is charged with a compressible gas which is compressed between the piston head and the inner surface of the free end of the cylinder as the piston is driven thereinto, and which is allowed to expand as the piston is withdrawn therefrom.
This particular arrangement is advantageous in a number of different respects. Firstly, when the pump piston is in its fully inserted condition within the pump cylinder, and the pump is used in conjunction with a telescopic cylinder to force fluid into said telescopic cylinder to effect hydraulic work, the compressible gas in the pump cylinder is in a state of maximum compression because the pump piston head is almost contiguous with the inner surface of the pump cylinder free end. Hence, the compressed gas exerts a restoring force on the pump piston and aids the withdrawal of the pump piston from said pump cylinder. The xe2x80x9ckick-startxe2x80x9d which the pump piston is given by the compressed gas reduces the requisite power rating of the motor needed to drive the spindle or race and effect hydraulic work.
Furthermore there is no requirement for a more powerfully rated motor to put work into the compressible gas to compress same when the piston is being inserted within the cylinder because when the pump is used in conjunction with a system which in naturally relaxing, for example under gravitational effects, urges hydraulic fluid back into the pump, the extra work required to compress the compressible gas is done during the natural relaxation of the system.
Preferably the outlet communicates with a cylinder of an hydraulically actuated device, and most preferably a closed system is provided such that actuation of the hydraulic device is effected when the piston is withdrawn from or urged into the cylinder.
Most preferably the pump is provided in communication with an hydraulic cylinder for actuation of a domestic elevator.
It is yet further preferable that the hydraulic fluid used is water. Preferably the pump is vertically orientated in use to minimise gravitational effects (which may cause the spindle to whip or hunt) on the spindle as it is rotationally driven.
It will be appreciated from the foregoing that a much improved hydraulic pump is provided, because firstly the rotational motion of the motor is translated into a linear motion by the ball screw and not directly to the fluid which ensures a smooth and steady fluid flow into and from the cylinder, and secondly because the fluid is water, fluid compressibility effects are drastically reduced.
The pump of the invention has a further advantage in that the ball screw provides gives a mechanical advantage and therefore there is no requirement for the driving motor to provide a large torque to drive the spindle.